Heat-shrinkable plastic films are used as coverings, to hold objects together, and as an outer wrapping for bottles, cans and other kinds of containers. For example, such films are used for covering the cap, neck, shoulder or bulge of bottles or for the entire bottles; for the purpose of labelling, protection, parceling, or increasing the value of the product; and for other reasons. In addition, such films may be used as a covering to package such objects as boxes, bottles, boards, rods, or notebooks together in groups, and such films may also be attached closely as a wrapping. The uses mentioned above take advantage of the shrinkability and the internal shrink stress of the film.
Shrink films can be classified into two categories as follows: (1) biaxially oriented film for over-wrapping wherein the film shrinks in both the x- and y-axis directions, and (2) uniaxially oriented sleeves which are widely used as tamper evident labels on food and pharmaceutical products and as primary labels on soft drink bottles. This film primarily shrinks in the stretched or oriented direction and has a little shrinkage in the unstretched or nonoriented direction. A 10 percent or less shrinkage in the unstretched direction is desirable to compensate the thermal relaxation of the shrink film during heating. Usually these films are made into a tube shape, and after being wrapped around a bottle or around more than one pipe, for example, the film is shrunk by the application of heat so as to wrap or to parcel together the object or objects.
The materials used for the films mentioned above include polyvinylchloride (PVC), polystyrene (OPS), oriented polyethylene, oriented polypropylene, hydrochlorinated rubber, and polyesters. PVC is the most widely used material due to its excellent shrink properties and clarity. However, environmental concerns exist with PVC. Replacement materials which are more environmentally friendly, have comparable heat-shrinkable properties and are substantially free of haze are desirous.
The prime choice to compete with PVC for shrink films has been copolyester blends of 1,4-cyclohexanedimethanol modified polyethylene terephthalate (known as PETG), a low glass transition temperature polyester or copolyester such as diethylene glycol modified polyethylene terephthalate (DEG modified PET) and, optionally, a crystallizable polyester such as polyethylene terephthalate (PET). The onset shrink temperature of these copolyester blends, while somewhat higher than the onset temperature of PVC, is close enough that existing shrink tunnels designed for PVC may be used. However, only up to 10 percent of the DEG modified PET may be used in the blends due to the occurrence of haze in the final heat-shrinkable film. The haze is caused by the high degree of yellow color in the DEG modified PET. Due to the nature of manufacturing technology, DEG modified PET, known commercially as EASTOBOND Copolyester 13162 available from Eastman Chemical Company of Kingsport, Tenn. has a b* color value (yellowness index) in pellet form of about 25. In order to increase the amount of DEG modified PET and possibly lower the onset shrink temperature of the polyester blends, a DEG modified PET would need to have a b* color value in pellet form substantially lower than conventional technology permits.
Thus, there exists a need in the art to have a heat-shrinkable copolyester film that has an onset temperature the same or lower than PVC film and does not produce undesired haze levels. Accordingly, it is to the provision of such that the present invention is primarily directed.